The use of X rays or gamma rays to view the internal structures of the body.

RADIOISOTOPES:

Radioactive elements.

RADIOTHERAPY:

Radiation therapy.

RAIN-SCALD:

Common term for dermatophilosis.

RECESSIVE GENE:

A gene that can be expressed only when both members of a chromosome pair contain the same allele for a given characteristic (i.e., the same allele must be inherited from both the dam and the sire).

RECOMBINATION:

Genetic exchange among chromosomes, producing new combinations of genes.

RECTAL-VAGINAL FISTULA:

A tear from the top of the vestibule (entrance to the vagina) to the floor of the rectum.

RECTUM:

Lowermost portion of the large intestine, immediately adjacent to the anus.

RECUMBENCY:

Inability to stand.

RECUMBENT:

Lying down.

RECURRENT UVEITIS:

Periodic ophthalmia.

REDIA:

Larval stage of flukes arising from the sporocyst stage.

REDUCTION:

The setting of a bone fracture.

REFLEX:

In general, muscle movement orchestrated by the nervous system in response to a stimulus and without conscious (voluntary) control; an example is the knee-jerk reflex.

REFLUX:

Backward flow.

REFRACTIVE:

Light-bending.

REFRACTOMETER:

Small hand-held device that can be used for determining how concentrated or dilute a urine sample is (a measurement referred to as urine specific gravity).

REGURGITATION:

Involuntary return of undigested food to the mouth after swallowing; differs from vomiting in that it is a passive process (i.e., unaccompanied by the reflex, propulsive movements characteristic of vomiting).

RENAL:

Pertaining to the kidneys.

RENAL PELVIS:

"Collecting funnel" deep within each kidney into which the kidney tubules drain filtrate.

RESERVOIR HOST:

An animal from which infection may be passed to domesticated stock or to human beings.

RESORPTION:

Biochemical dissolution or loss of tissue.

RESPIRATION:

Breathing.

RETCHING:

Abdominal contractions in preparation for vomiting.

RETICULATED LEUKOTRICHIA:

A form of leukotrichia seen primarily in Quarter Horses.

RETINA:

The light-sensitive layer of cells at the back of the eye.

RETINITIS:

Inflammation of the retina.

RETROGRADE:

Backward.

RHABDOMYOMA:

A benign tumor of striated muscle cells.

RHABDOMYOSARCOMA:

A malignant tumor of striated muscle cells.

RHINITIS:

Inflammation of the nasal passages.

RHINOPNEUMONITIS:

Respiratory condition of horses caused by equine herpesvirus.

RHINOSCOPY:

Visual examination of the nasal passages, using an endoscope, otoscope, or other instrument.

RHINOSPORIDIOSIS:

An uncommon disease caused by an as yet poorly characterized fungus, Rhinosporidium seeberi; it is a chronic localized infection characterized by the formation of polyps (fleshy masses protruding from the surface of a mucous membrane) in the nasal passages.

RHYTHMIC SEGMENTATION:

Rhythmic muscular movements of the intestinal tract that serve to delay the passage of intestinal contents until digestion and absorption have been completed.

RIBONUCLEIC ACID (RNA):

A nucleic acid occurring in all cells and involved in cell division, gene expression, and protein synthesis; also serves as the genetic material for some viruses.

RICKETTSIAE (SINGULAR: RICKETTSIA):

Specialized bacteria that multiply only within host cells and that are usually transmitted to animals or human beings by lice, ticks, fleas, or mites; examples include Ehrlichia risticii, the cause of Potomac horse fever (equine monocytic ehrlichiosis), and Ehrlichia equi, the cause of equine granulocytic ehrlichiosis.

RINGBONE:

Disorder characterized by new bone growth adjacent to either the pastern or coffin joints; caused by tearing of the collateral ligaments stabilizing the joint. High ringbone describes bone growth around the pastern joint, while low ringbone describes bone growth around the coffin joint.

RINGWORM:

A common skin infection caused by ringworm fungi (dermatophytes), which invade the outer, superficial layers of the skin, hair, and nails; also called dermatophytosis.

RNA:

See ribonucleic acid.

ROARING:

Common name for laryngeal hemiplegia.

RODS:

Photoreceptor cells in the retina of the eye that are responsible for night vision and detection of motion.

ROOT:

The portion of a tooth that lies below the gumline.

ROOT CANAL:

The chamber within each tooth that contains nerves, blood vessels, and lymphatic channels; also known as the pulp chamber.